1. Technical Field
The present invention relates to methods of manufacturing nitride semiconductor devices such as light-emitting devices; more specifically, the present invention relates to methods of manufacturing high-quality nitride semiconductor devices excelling in flatness and manufacturable at high yields.
2. Description of the Related Art
In the manufacture of light-emitting and other semiconductor devices, when vapor-phase epitaxial film growth is carried out with a nitride-based single crystal as a substrate, the substrate is annealed with the object of ridding the substrate of contaminants (organic matter, moisture) adhering to it, and defects (scratches, strain, etc.). Inasmuch as this heating process is performed prior to the epitaxial film growth, the process is called “pretreating,” “preheating,” “cleaning,” “purification,” or simply “annealing.”
To date—and this is not limited to nitride semiconductors—the substrate heating temperature in removing the contaminants and defects has been made at or above the temperature to which the substrate is heated in the deposition process for growing an epitaxial film onto the post-treated substrate (patent documents including Japanese Unexamined Pat. App. Pub. Nos. 2000-174341 and 2000-323752). The motivation behind this is that it has taken over from a procedural operation, in methods of manufacturing silicon-based semiconductor devices, in which the substrate temperature for cleaning is made to be at or above the film-deposition temperature of the substrate—with cleaning being stressed, because the higher the substrate heating temperature in pretreating is made, the better cleaning will progress. In the manufacture of nitride-based semiconductor devices also, likewise as with silicon-based devices, the substrate surface can be cleaned by making the temperature to which the substrate is heated in the pretreating process be at or above the temperature to which the substrate is heated in the film-deposition process.
For nitride-based semiconductors, because the vapor pressure of nitrogen is high, when nitride single-crystal substrates are to be annealed at high temperature, nitrogen or an atmosphere containing atoms from the same group as that of nitrogen—Group VA in the periodic system—is used. An atmosphere from which source-material gas containing the Group IIIA element has been omitted from among the ambient gases that are introduced in the film-deposition operation is generally employed as the annealing atmosphere. The reason for this is because nitrogen tends to come loose from the surface part of the substrates, leading to surplus Group IIIA element there, which is liable to produce roughness in the surface of a nitride semiconductor substrate. Accordingly, to keep roughness originating in the accumulation of surplus Group IIIA element on the substrate surface from occurring, arrangements are made for an atmosphere as just noted that does not contain Group IIIA element, and the escape of nitrogen from the substrate is controlled.
No matter what the atmosphere employed, as discussed above, in conventional annealing, in order to enhance the cleaning effectiveness the substrate is heated to at or above the heating temperature during film deposition. In the case of nitride-based semiconductors, nevertheless, heating the nitride substrate for semiconductor device fabrication to at or above the temperature to which it is heated during film deposition provokes the breaking away of nitrogen from the substrate surface or the decomposition of ammonia (NH3), which renders necessary considerable labor for optimizing the gas-supply parameters—in which there are immense options. As an extreme example, carrying out pretreatment under the same gas-supply conditions as during film deposition ruins the planarity of the nitride semiconductor substrate and, with the epitaxial film formed during the film deposition not being flat, leads to three-dimensional growth. The consequent problem has been that growing epitaxial films using nitride single-crystal substrates has meant that the gas-supply requirements in the foregoing substrate cleaning process have to be sought out by trial and error